It is known in the art to obtain trays by thermoforming techniques, especially vacuum moulding, and to stack them one into the other. It is also known to stack trays defining an egg-box into an opened position (see Applicant's Canadian Patent No. 2,028,229). However, when trays are stacked one into the other, friction and/or air lock maybe created between neighboring trays. Such interlocking of trays involves that when a tray is picked up (or denested) from the stack, one or several neighboring trays may be simultaneously picked up. This drawback becomes very important when said stack of trays is intended to feed an automated packaging and/or labeling apparatus, especially an apparatus intended to fill egg-box trays with eggs. Indeed, when the apparatus becomes jammed, it has to be stopped and an operator must manually remove the trays from the apparatus. There is a substantial lost of productivity and of course substantial risks of damaging the apparatus.
It is also known in Applicant's co-pending U.S. application Ser. No. 10/934,400 which is hereby incorporated by reference to obtain stackable tray by vacuum molding of a sheet of plastic material, said tray comprising, in open position:                a) a top and a bottom;        b) at least one receiving cavity opened upwardly; and        c) spacing means to keep, in a stack of trays, the top portion and the bottom portion of neighboring trays at distance from each other to thereby prevent interlocking between adjacent trays.        
However, when such a tray defines an egg-box of large size, for example for 18 eggs or more, it appears that said box resulting from the folding of a cover portion over a portion provided with egg receiving cavities, does not present a level of rigidity allowing to prevent deformation of the box and to optimize the protection of its fragile content.
Therefore, there is a need for trays of the type as defined in co-pending U.S. application Ser. No. 10/934,400 that can be stacked one into the other without creation of air lock or friction between neighboring trays, that can allow an easy removal of each tray from the stack of trays and whose box resulting from the folding of a cover portion over another portion provided with egg receiving cavities, will present a sufficient rigidity to protect adequately its fragile content.